Metal halide discharge lamps are favored for their high efficacies and high color rendering properties which result from the complex emission spectra generated by their rare-earth chemistries. Particularly desirable are ceramic metal halide lamps which offer improved color rendering, color temperature, and efficacy over traditional metal halide lamps having quartz discharge vessels. This is because ceramic discharge vessels can operate at higher temperatures than their quartz counterparts and are less prone to react with the various metal halide chemistries.
Most commercial ceramic metal halide lamps contain a fill comprising an amount of mercury and a complex combination of metal halides, particularly iodides. The fill chemistries of metal halide lamps are carefully selected to achieve a white light emission having a high color rendering index (CRI) and a high efficacy (lumens/watt, LPW). The condensation and evaporation behavior of the fill materials within the discharge vessel (also commonly referred to as an arc tube) affects the performance of the lamps over their operating life. A lack of control over this behavior can lead to unpredictable changes in the lamp's correlated color temperature (CCT) or CRI. Thus, it is desirable to have a well-defined region for the condensate to form in the discharge vessel so that the metal halide lamps have more stable color characteristics.
Various means for controlling the condensation-evaporation behavior of metal halide and similar metal amalgam fills have been developed for use with different types of HID lamps. For example, reflective coatings, in particular ZrO2, have been applied on the outside surface of quartz arc tubes in metal halide lamps, and metal heat shields have been wrapped around the cold ends of small-wattage high-pressure sodium (HPS) arc tubes. For ceramic metal halide lamps, prior-art approaches to achieve a well-defined, small-size cold spot, include the use of asymmetric electrode feedthroughs and minimizing the space between the feedthrough and wall of the discharge vessel.